Automatic volume control circuit



Feb. 1.7, 1 942. D. E. FOSTER 2,273,093

AUTOMATIC VOLUME CONTROL CIRCUI'IVI Filed oet. so, 1937 ik. N

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ATTORNEY Patented Feb. 17, 1942 UNITED STATES TNT OFFICE AUTOMATIC VOLUME CONTROL CIRCUIT Dudley E. Foster, South Orange, N. J., assignor to Radio Corporation of America, a corporation of Delaware 8 Claims.

My present invention relates to automatic gain control circuits for radio receivers, and more especially to automatic volume control circuits which function to maintain the -receiver output substantially uniform.

One of the main objects of my present invention is to provide an automatic volume control circuit for a radio receiver, and wherein the control circuit employs, between the signal collector and the high frequency amplifier, a tube having a network between its input electrodes such that signal energy is opposed to a degenerative potential, the resultant being fed to the high frequency amplifier, and the magnitude of the resultant potential being under the control of the carrier amplitude.

Another important object of my invention is to provide a method of controlling the volume of a radio receiver, which method -includes the step of utilizing received signals to develop a potential in degenerative phase, feeding the resultant through the receiving system, and controlling the magnitude of the resultant potential in response to changes in carrier amplitude.

Still other objects of the invention are to improve generally the simplicity and efiiciency of' automatic volume control circuits for radio receivers, and more especially to provide circuits employing degenerative action, and which circuits are not only reliable in operation, but economically manufactured and assembled in radio receivers. c

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

Referring now to the accompanying drawing, wherein is shown a conventional superheterodyne receiver employing the invention, it will be observed that the receiver comprises the usual networks. The signal collector I is employed to collect modulated signal energy, and it is to be understood that the modulated signals may be included in the broadcast band of 500 to 1500 kc. Of course, if the receiver is of the multi-wave type, then the collected signals will be included over these various bands. The collector I maybe of the grounded antenna type; a radio frequency distribution line; a loop collector, or even a collector employed on a mobile structure. The

collected signals are transmitted, through a network to be described at a later point, to the tunable radio frequency amplifier 2, and then to an I. F.. (intermediate l frequency) amplifier 4 through a converter network 5. The converter comprises the usual tunable first detector and local oscillator circuit, and it will be understood that the reference numeral 6 denotes the usual tuning condenser which is employed in the signal and local oscillator tank circuits.

The output of the I. F.l amplifier 4 is impressed upon a diode demodulator 1 through an I. F. transformer 8, whose primary and secondary circuits are each fixedly resonated to the predetermined operating I. F. value. The demodulator 1, which is the second detector of the receiver, includes the Vusual load resistor 9 in series between thel cathode and the grounded side of the demodulator input circuit, the resistor 9 being shuntedby I. F. by-pass condenser I0. The modulation component of the voltage developed across resistor is impressed upon one or more audio frequency amplifiers through -an audio frequency transmission path I 2. Of course, the final audio amplifier will feed its output energy to any desired form of reproducer.

It is to be clearly understood that the receiver of the circuit need not necessarily be of the superheterodyne type; it may be ofthe tuned radio frequency type. In that case, the various radio frequency amplifiers will have their input circuits tunable in unison to varioussignal frequencies in the operating frequencyranlge. In order to maintain the signal carrier amplitude at the demodulator input circuit substantially uniform regardless of a wide range of carrier amplitude variation at the collector I, there is customarily employed an automatic volume control (AVC) circuit.` In the past, such AVC action has been accomplished by employing the direct current voltage component of rectified signals of one or more signal ampliers. In such type of AVC circuit, as thecarrier amplitude increases the signal grids of the controlled amplifiers are biased increasingly negative thereby to reduce the gain in each amplifier.

AIn the present arrangement AVC actionvis secured by a form of signal-controlled degenera..

grid of tube I I, which may be of the pentode type, are connected to sources of positive potential, and it will be observed that the plate and screen grid of tube I6 are also energized from positive potential points. The plate of tube I6 is connected to its point of positive potential through a path which includes the coil I'I and resistor I8. The coil II is the primary winding of transfo-rmer I9, the secondary of the latter being in the tunable input circuit of the radio frequency amplifier 2. Of course, a common source of positive potential can be used to energize the screen grid electrodes and plate electrodes of tubes II and I6.

The control grid I3 of tube Il is connected to the cathode side of load resistor 9 through a path which includes resistor 20, resistor 2| and lead 22; the lead 22 is designated by the reference letters AVC to denote that this is the automatic volume control connection. The junction of resistors 20 and 2| is connected to the grounded side of resistor I2 through a condenser 23. The cathode side of resistor I2 is connected to the junction of resistor 24 and coil 25 through a condenser 26; the junction of the latter two elements is connected to the cathode resistor 2'I of tube IG, the resistor 21 being shunted by a signal frequency by-pass condenser 28.

One terminal of the series path including resistor 24 and coil 25 is connected to the grid. side of condenser I4, whereas the other terminal of the path is established at ground potential. Appropriate by-pass condensers are employed to bypass the energizing leads connected to the positive electrodes of tubes II and I6.

Resistor 2U is the impedance across which the antenna to ground (which is the same as grid I3 to ground) voltage is developed. Resistor 2I is a lter resistor; this resistor in conjunction with capacity 23 allows the direct current voltage developed across resistor 9 to be impressed on grid I3, but prevents any of the alternating voltage components across resistor 9 from being impressed on grid I3. Resistor 24 supplies the bias voltage (due to the drop across resistor 21) to tube I6. Resistor IB is a radio frequency isolating resistor so that the R. F. (radio frequency) from the plate circuit of tube I6 returns to cathode through the condenser 29 and condenser 28, rather than through the B supply. If it returned through the B supply, choke 25 and resistor I2 would be short-circuited as far as R. F. was concerned and no degeneration would occur. A choke similar to 25 could be used in place of resistor IB.

Coil 25 is an R. F. choke to present a low resistance D. C. (direct current) path for the plate current oitube I6 to return to the cathode thereof, but presents a high impedance to R. F. currents` in the grid circuit so that the input R. F. will flow from the cathode of tube II through condenser 26 to tube I6 and thus give the desired degenerative action. If choke 25 were absent the signal energy between grid and ground would be applied directlyto the input of IE, and not be controlled by tube I I.

The signal current in the antenna circuit follows the following path: antenna I, resistor 20, condenser 23 to ground. Condenser 23 provides a low impedance path for R. F., but blocks the D. C. so that the bias of grid I3 is the D. C. developed across resistor 9 by the signal, and not the D. C. across resistor I2. Condenser 23 is thus in the antenna-ground path and the gridcathode path of tube I I. It should be sufliciently large, say of the order of 0.1 mf. (microfarads).

It functions as a blocking and by-pass condenser. The ground connection for antenna I is the ground at the bottom of resistor I2. A separate ground at the bottom of resistor 20 could be provided, but should be isolated to D. C. by a condenser. Since condenser 23 is already present, grounding the antenna at the point shown makes capacity 23 doubly useful.

In operation, when the signal carrier amplitude increases, the cathode side of resistor 9 will increase in positive potential with respect to ground. As a result, the signal grid I3 will be biased in a positive sense, and cause the tube II to become conductive. It will be understood that the resistor I2 has a magnitude such that there is substantial cut-olf bias applied to grid I3 in the absence of signals. Hence, when signals are received, the tube II increases in conductivity and current flows through resistor I2. This results in the signal intensity at the input of tube I6 being reduced.

This can be readily understood when it is realized that there is impressed upon grid I3 signal voltage in degenerative phase by virtue of the voltage drop across resistor/I2. When weak signals are being received, then the signal potential diierence between the control grid I3 and ground is a maximum. As the signal intensity increases, however, the mutual conductance of tube II increases by virtue of the positive potential impressed on grid I3 through lead I2, and causes an increasing flow of current through resistor I2. Since the voltage across I2 is impressed upon grid I3 in degenerative phase with respect to the signal potential impressed thereon from the collector I, it follows that an increase in carrier amplitude will cause the effective signal potential diierence between grid I3 and ground to be decreased. The development of degenerative voltage across resistor I2 is chosen so that the signal carrier amplitude at the demodulator input circuit will be substantially uniform.

The effective voltage at the control grid I3 of tube I I is impressed upon the input electrodes of tube I6, and the amplified output of tube I6 is transmitted through the following stages. It will, therefore, be seen that the potential difference between the control grid and cathode of tube I I varies inversely with the mutual conductance, or transconductance, of tube II, and the control of the latter is developed at the detector load resistor. Hence, when the mutual conductance of tube II is high, the potential difference between the input electrodes of tube II is low. The arrangement shown has the advantage in that there will be freedom from cross-modulation, because when the signal input to the receiver is large, the tube I I will be operating on the straight portion of its characteristic.

More specifically, the operation is as follows: In the absence of signals the D. C. plate current of tube II iiows into the B supply and returns to the cathode through resistor I2 and develops a bias thereacross, which bias is applied to grid I3 by way of resistors 9, 2I and 20. With this bias, which can be made close to cut-off bias of tube I I, substantially all of the signal voltage developed across resistor 20 will likewise appear between grid and cathode of tube II, and, therefore, be transmitted to the input of amplier tube I6. This is under conditions of small signal on the antenna, which might, for example, develop 0.5 volt across resistor 9. This small positive voltage will cause only a small increase in mutual conductance of tube II, and, therefore, will produce only a small amount of degeneration. T'his small change in gain vat small signals is more true on this type of AVC than on the conventional type because the controlled tube (in this case tube II) is operating near cut-off, and the curvature of the tube characteristic in that region means that an appreciable change in bias voltage is necessary before the mutual conductance changes greatly. With larger signals applied, voltages are developed across resistor 9 of sucient magnitude to cause an appreciable change in mutual conductance of tube I I. When the mutual conductance of tube I I is high a large R. F. voltage will appear across resistor I 2 which opposes, or degenerates, the input R. F. voltage so that the net voltage appearing between grid and cathode of tube II is decreased from the magnitude applied to the input of that tube. The grid-cathode voltage of tube II is applied to the amplifier input (input to tube I6), and, therefore, changes in mutual conductance of tube II serve to control the signal applied to grid of tube I6. As shown in the figure this control may be made automatic by using the D. C. developed by the second detector.

In order to illustrate the constants of the circuit which may be used, and it being understood that such constants are in no way restrictive, the following table of magnitudes is given:

Resistor 20:5000 ohms Resistor 21=500,000 ohms Resistor 12=50,000 ohms Resistor 18=50,000 ohms It is to be understood that a tunable input circuit may be connected between the input electrodes of tube I6, if desired. Further, the condenser 26 and coil 25 may be omitted if the change in bias on grid I3 of tube II, under AVC action, does not change the operating potential of tube I6 too greatly.

While I have indicated and described one system for carrying my invention into eiect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In a radio receiver,asignal transmission tube, means for applying signal energy between the control grid and cathode of the tube, an output circuit connected directly between the control grid and cathode of the tube, an impedance disposed in the space current path of said tube, means connecting said control grid and cathode of said tube across points of said impedance such that the signal voltage developed across the impedance is applied in degenerative phase between the two electrodes, and means automatically responsive to variations in amplitude level of received signals for varying the gain of said tube thereby to vary the signal voltage amplitude delivered to said output circuit.

2. In a radio receiver, a signal transmission tube, means applying signals between the input electrodes of the tube, a second tube, a network connected directly between the input electrodes of the rst tube and the input electrodes of the second tube, an impedance disposed in the space current path of the first tube, said input electrodes of the rst tube being connected to points on the impedance such that signal voltage developed across the impedance is impressed in 3 degenerative phase between the input electrodes of said rst tube, and means for varying the gain of said first tube in response to variations in carrier amplitude of said received signals.

3. In a radio receiver, a tube having at least a cathode, grid and cold electrode, an antenna including a resistive impedance connected between the grid and cathode, impedance means in the space current path ofV said tube for deriving a voltage from signal voltage impressed between said grid and cathode, means for impressing said derived voltage in series with said resistive impedance in degenerative phase to the impressed signal voltage, a second tube having input and output electrodes, and means connecting the second tube input electrodes directly across said resistive impedance.

4. In a radio receiver, a tube having at least a cathode, grid and cold electrode, an antenna including a resistive impedance connected between the grid and cathode, impedance means in the space current path of said tube for deriving a voltage from signal voltage impressed between said grid and cathode, means for impressing said derived voltage in series with said resistive impedance in degenerative phase to the impressed signal voltage, a second tube having input and output electrodes, means connecting the second tube input electrodes directly across said resistive impedance, and means, responsive to an increase in signal carrier amplitude, for increasing the magnitude of said derived voltage.

5. In a wave receiving system, a tube having at least a cathode, a grid and a plate, an impedance connected between cathode and ground, a source of waves connected between the grid and ground, a wave utilization network coupled solely between said grid and cathode, said impedance developing thereacross voltage of wave frequency, means impressing said developed Voltage upon said grid in degenerative phase, and means for automatically increasing the gainl of said tube as the wave amplitude increases thereby to increase the magnitude of the degenerative voltage.

6. In a modulated-carrier signal receiver comprising an input circuit and a vacuum tube having an input circuit in the signal-translating channel of the receiver, an attenuator comprising a coupling circuit for coupling said input circuit of said receiver to said input circuit of said vacuum tube, a cathode-resistor for said vacuum tube, said vacuum tube having an output circuit returned for signal-frequency components directly to its cathode, substantially nonfrequencyselective means for developing across said resistor potentials opposite in phase to those developed in said vacuum-tube input circuit by said coupling circuit, and means responsive to the intensity of received signals for varying the effectiveness of said last-named means directly in accordance therewith.

7. In a modulated-carrier signal receiver comprising an input circuit and a vacuum tube having an input circuit in the signal-translating channel of the receiver, an attenuator comprising a first coupler circuit for coupling said input circuit of said receiver to said input circuit of said vacuum tube, a cathode resistor for said tube, said vacuum tube having an output circuit returned for signal-frequency components directly to its cathode, a second coupling circuit having an output circuit including said resistor and developing potentials across said resistor opposite in phase to those developed in said vacuum-tube input circuit by said rst coupling coupled between said linput circuit ofxsaidv. receiver and saidinput circuit of said tube, a-common cathode-biasing resistor for said tube and said Vacuum-tube signal repeater, said vacuum tube having an output circuit returned for signal-frequency components directly to its cathode, said resistorr comprising substantially the entire load impedance of said vacuum-tube signal repeater and being effectively by-passed for alternating currents in the anode circuit of said vacuum tube, and means for controlling the transconductance of said repeater in accordance with the amplitude of received signals.

DUDLEY E. FOSTER. 

